Thanks to TFP, I recently fixed an air leak which was causing a failure to prime, but now I have a new quandary, which may or may not have anything to do with my pump, which I assume is very old. Anyway, after vacuuming, as expected, filter pressure rose to the point of low return--smile if you now have a Kansas song playing in your head--about 25 PSI. Since it was getting dark, I decided to backwash the next day when I would be able to see the water viewing glass more clearly. To my surprise, the pressure the next day was less than 20 PSI. I backwashed anyway since clean pressure is well under 15 PSI and added DE, bringing my pressure to 14.5 PSI. The pool was cloudy after the first vacuum since the pool floor was pretty dirty and a lot of debris didn't make it into the vacuum and got stirred up instead. That being the case, several days later, the water was clear again and I could see I had more vacuuming to do. This time, pressure rose to about 21 PSI. Again, due to darkness, I postponed the backwash until the next day. Once again, I found the pressure to have decreased by about 5 PSI, to about 16 PSI. (It was actually initially around 15, but gradually rose to 16 during the hour or so it was on.) Also, this is somewhat subjective, but the force of water at the returns seems to be weaker than it should be with the pressure only marginally higher than right after backwashing and adding DE. (It is, at least, noticeably stronger at the returns than it was at 21 PSI before I turned off the pump right after vacuuming.) This seems very strange. Is the debris on the surface of the sand doing some sort of settling when the pump is off to cause this decrease in pressure? Ciould it have anything to do with the old pump? Thanks in advance for any ideas.